1. Field of the Invention
The present invention generally relates to halftoning techniques in printers, and more particularly to an apparatus for halftoning which combines advantages of digital printing and offset printing.
2. Background Description
Most printers today can print in only a limited number of colors. Digital halftoning is a technique for printing a picture (or more generally displaying it on some two-dimensional medium) using small dots with a limited number of colors such that it appears to consist of many colors when viewed from a proper distance. For example, a picture printed with black and white dots may appear to contain various shades of gray when viewed from a distance.
The fastest and most commonly used methods for digital halftoning are dithering algorithms which use threshold arrays, also called dither matrices or dither masks. The principle of this method, as illustrated in FIG. 1, is well known. In the case of greyscale images, the method allows one to generate a matrix N of discrete values at 113 (typically 0 or 1, where 1 means a pixel is printed and a 0 means nothing is printed) from an image I at 111 using a dithering mask (a smaller matrix of threshold values) at 112. This process involves comparing at 114 a specific image value with a specific mask value, and then generating a 1 if the image value is greater than the mask value and 0 otherwise. Various masks can be devised, according to the needs of precise applications, and several methods to devise masks with good performance have been disclosed. See, for instance, U.S. Pat. Nos. 5,111,310 to Parker et al. and 5,917,951 to Thompson et al. and pending U.S. application Ser. No. 08/909,535 to Thompson et al.
Dithering algorithms are used both in digital printers (such as laser printers and ink jet printers) and analog printers such as offset printers.
In a color printer, the color is decomposed into fundamental colors, usually cyan (denoted C), yellow (denoted Y), and magenta (denoted M), and black (denoted K). During printing, several passes are used, one per color. More recently, other colors may be used. (For simplicity of presentation, the most common case of four color (CMYK) printing will be assumed, although one skilled in the art can appreciate that the present invention may easily be adapted to other or more general cases.)
In traditional (analog) color printing, where halftone dots are placed at will on the mask, the same mask is employed for all colors, except that each color mask is given a different angle in order to avoid moire patterns. This results in small rosettes which constitute the fundamental small scale texture of all offset prints.
In the case of digital printing, where all pixels are points on a square or rectangular lattice, some rotations of a given mask are incompatible with the lattice. Optimal choices of rotation angles for CMYK are 75 degrees, 15 degrees, 90 degrees, and 45 degrees, respectively. Of these angles, only the 90 and 45 degree rotations can be implemented on a square lattice. The commonly used 15 degree rotation of an analog mask cannot be aligned with a square lattice. Hence, in the digital domain, one has to compromise and choose and angle that is close to 15 degrees and yields perfect alignment with a square lattice. For instance, an angle of 14.036 degrees (obtained by a displacement of 4 units along the x-axis and a displacement of 1 unit along the y-axis) yields perfect alignment with a square lattice.
Because non-optimal angles must be used, the output quality in digital printers suffers from more noticeable moire patterns than is evident in conventional analog printers. Furthermore, different masks must be generated for each angle. This is a considerable problem, as generating even a single mask with good performance has proven difficult. These drawbacks are discussed in detail, for example, in the text Optical Color Technology for Electronic Imaging Devices by Kang, Henry R. (Bellingham, Wash., USA:SPIE Optical, 1997), which is mentioned here as a general reference for color digital printing.
In U.S. Pat. No. 5,146,242 to Zielinski, a device for controlling the angular alignment of a writing device is disclosed.
3. Problem to be Solved
In view of the foregoing considerations, it would be advantageous to combine the benefits of digital printing (such as flexibility and low cost for a small number of prints) with the benefits of analog printing without realizing their disadvantages, and more specifically to do so by using a single mask at several angles. The present invention offers a way to combine these benefits in a single apparatus.
It is an objective of the present invention to provide a printing apparatus which performs digital printing (for instance, laser or ink jet) using a single mask which may be rotated at different angles, thereby forming a semi-digital printer which realizes the benefits of digital printing and analog printing without achieving their disadvantages.
It is another objective of the present invention to achieve the aforementioned objective by providing a semi-digital printer having a print head assembly which is aligned relative to a sheet of paper to be printed on, instead of in a predetermined, fixed position.
It is another objective of the present invention to provide a semi-digital printer which orients its print head assembly in such a manner that each print head prints an orthogonal array of dots at different angles of rotation relative to the paper, the different angles of rotation being achieved by rotating either the paper or the print head assembly. The orthogonal array of dots printed by each head at its respective angle of rotation advantageously allows for the use of a dither mask, for example, to generate, for example, a halftone or color image which is oriented precisely at the chosen angles, a feature which is not provided by conventional digital printers.
These and other objectives are achieved by providing a semi-digital printer which, according to a first embodiment, includes: a print head assembly that prints an image on a sheet of paper mounted onto a turntable; and a rotating means which rotates the turntable to at least one predetermined angle relative to the print head assembly. In operation, the rotating means rotates the turntable so that the at least one print head in the print head assembly ejects ink or toner onto the sheet of paper at the predetermined angle. Preferably, the print head assembly includes multiple print heads, each of which prints either the same color of ink or toner (e.g., black) or different colors (CMYK). When multiple print heads are included, the rotating means advantageously rotates the turntable at different predetermined angles, one for each color. By rotating the turntable in this manner, the printer of the present invention records an image at any angle between xe2x88x9245xc2x0 and +45xc2x0 precisely (i.e., without having to perform the approximation required by conventional digital printers at, for example, 15xc2x0 and 75xc2x0), thereby producing a printed image of improved quality. Further, to increase the speed of printing, a look-up table may be used to pre-store rotational coordinates at various angles. The rotating means may then access the look-up table when printing is required.
A second embodiment of the printer of the present invention is similar to the first embodiment, except that in the second embodiment the print head assembly is mounted on a turntable and rotated by a rotating means relative to a sheet of paper.